In theory, the total-internal-reflection lens can collect all the rays from the source, thus it is the preferred structure for secondary optical design in long-distance collimating illumination systems. The illumination characteristics of a collimating illumination system with extended source based on this type of light distribution element, including the intensity distribution characteristic, spot illuminance uniformity, and flux utilization, are analyzed by using optical software and experiment measurement, and the determining mechanism of divergence characteristic of the outgoing beam is explored. It is found that compared with the light distribution effect of the same-aperture plano-convex collimating lens, the flux utilization of the total-internal-reflection lens is improved significantly. However, the divergence angle of the outgoing beam is also remarkbly increased, which leads to no essential difference in the outgoing beam intensities of these two light distribution elements. Because the divergence angle of the outgoing beam from the core refraction part is different from that from the edge total-internal-reflection part, the illuminance uniformity of illumination spot for the total-internal-reflection lens is obviously deteriorated.